The present invention relates to the calibration of the time axis of an X-ray streak tube when an X-ray pulse with an extremely short duration of time is generated by a pulse mode X-ray tube with the photocathode of the pulse mode X-ray tube responding to a laser beam pulse having an extremely short duration of time.
Conventional method for calibrating the time axis of an X-ray streak tube will be described referring to FIGS. 1 and 2.
FIG. 1 shows a block diagram of an example of a device to calibrate the time axis of an X-ray streak tube in the conventional way.
FIG. 2 is a graph of an X-ray spectrum obtained from an X-ray target to generate X-rays with an extremely short duration of time, which can be used in the device to calibrate the time axis of the X-ray streak tube in the conventional way.
A laser pulse signal with an extremely short duration of time goes into two paths, one passing through half mirror 3 and the other reflected from the half mirror 3. The laser pulse signal along the former path goes through etalon 2 where a train of light pulses 4 with a certain time interval can be generated.
An aluminum (Al) or titanium (Ti) target 14 is exposed to a train of light pulses 4 so as to generate a laser plasma. A train of soft X-ray pulses resembling the light pulses can thus be generated.
FIG. 2 shows the spectrum distribution of the soft X-ray pulses when target 14 is made of titanium (Ti).
Photocathode 10, which is of the X-ray excitation type, in the X-ray streak tube is exposed to a train of X-ray pulses.
Photocathode 10 of the X-ray excitation type is stimulated by a train of soft X-ray pulses 15 so as to emit electron beam 12.
The other light pulse reflected from the half mirror 3 is detected by high speed light detector 5.
Deflection voltage generator circuit 7, which generates a sweep voltage for scanning the X-ray streak tube, is used to apply the sweep voltage to deflection electrode 9 of the streak tube 27 when the deflection voltage generator circuit 7 is triggered by the output of the high speed light detector 5.
Electron beam 12 emitted from the photocathode 10 of X-ray excitation type is deflected by the deflection field across the deflection electrodes 9.
When the light pulse image is created on the phosphor layer 11 in accordance with the train of the X-ray pulses incident onto the photocathode 10 of the X-ray excitation type, the time axis of the streak tube can be calibrated by measuring both interval 16 and half-value width 17 of the light pulse image on the phospor layer 11.
The soft X-rays generated by such laser plasma as used in the device to calibrate the time axis of the X-ray streak tube shown in FIG. 2, have an energy distribution covering up to 1.5 KeV to 2.0 KeV. The energy covers the wavelength ranging from the VUV rays to the soft X-rays.
Each soft X-ray pulse is delivered in accordance with a time sequence which is different from the incident laser pulse. In other words, the laser plasma is created on the target which has absorbed the laser energy. The wavelength distribution of the electromagnetic waves (VUV through soft X-rays) radiated from the target can be determined depending on the plasma temperature, and the time sequence can be determined depending on the transient characteristics of the plasma. During radiation in the VUV region, for instance, a long time constant is observed.
A threshold exists in laser energy to generate the soft X-rays. That is, soft X-ray pulses cannot be so stable as to calibrate the time axis.
The objective of the present invention is to provide apparatus for calibrating the time axis of an the X-ray streak tube using an improved X-ray source providing the capability to accurately calibrate the time axis of the X-ray streak tube.